The present invention relates to image forming apparatuses and methods of measuring linear speed.
In an electrophotography process type image forming apparatus, the following operations are performed. An electrostatic latent image is formed on a surface of a photosensitive member using an exposure device. Then, using a developing device, toner is adhered to the electrostatic latent image, so that toner development can be performed. The toner image that is formed is then transferred to an intermediate transfer belt. From the intermediate transfer belt, the toner image is then transferred to a sheet, and is fixed to the sheet.
In such an image forming apparatus, in order to precisely transfer the toner image to the intermediate transfer belt and the sheet, the linear speed of the intermediate transfer belt is measured, and is used in various control operations.
Examples of methods of measuring the linear speed of the intermediate transfer belt are: (a) fixing a plurality of marks in an area of the intermediate transfer belt to which a toner image is not transferred (the marks having reflectivities that differ from the surfaces of other areas of the intermediate transfer belt), using a sensor to optically detect the passage of these marks at a predetermined detection position, and calculating the linear speed of the intermediate belt using the difference between the time at which the marks pass the detection position and the intervals between the marks on the intermediate transfer belt (first method); (b) fixing a plurality of holes in an area of the intermediate transfer belt to which a toner image is not transferred, using a sensor to optically detect passages of these holes at a predetermined detection position, and calculating the linear speed of the intermediate transfer belt using the difference between the time at which the holes (marks) pass the detection position and the intervals between the holes in the intermediate transfer belt (second method); and (c) setting an encoder at a driven roller upon which the intermediate transfer belt is placed, using the encoder to measure the rotational speed of the driven roller, and calculating the linear speed of the intermediate transfer belt using the rotational speed of the driven roller (third method).
In another technique, in certain image forming apparatus, a sensor for detecting toner density includes a plurality of light-emitting elements and a plurality of photodetectors. The light-emitting elements and the photodetectors are disposed perpendicularly to a direction of movement of the intermediate transfer belt The position of a toner mark in a direction that is perpendicular to the movement direction of the intermediate transfer belt is detected using the outputs of these photodetectors.
However, in the first and second methods, the intervals between the marks and holes, formed in the intermediate transfer belt, can change due to environmental conditions and over time. As a result, the linear speed of the intermediate transfer belt may not be exactly measured. In the third method, slippage between the driven roller and the intermediate transfer belt can occur and change due to environmental conditions and over time. As a result, the linear speed of the intermediate transfer belt may not be exactly measured.